RESPIRATION IN AIR 59 



reducing the rate of evaporation while allowing just the 

 necessary diffusion of oxygen and C0 2 . During muscular 

 movements the spiracles are opened and remain open for a 

 short time (1/4-2 m) after vigorous struggling. Hazelhoff 

 showed that C0 2 is responsible for this opening, which 

 amounts to about 1/10 of the maximal in 1% C0 2 and 1/2 in 

 2]/ 2 % C0 2 . Special experiments showed (as in insects, p. 

 119) that local mechanisms at each spiracle sensitive to C0 2 

 are responsible and affect only that particular spiracle. Ex- 

 periments with low oxygen tensions showed that down to 5% 

 2 the spiracles would remain almost closed, and even so the 

 diffusion is evidently sufficient. Lack of oxygen would give 

 rise to an extra liberation of C0 2 which would cause the 

 spiracles to open up, as it was observed at still lower oxygen 

 concentrations. 



In the air-breathing snails and slugs, both in the truly 

 terrestrial and in the fresh-water forms, the mantle cavity is 

 developed as a lung. This opens to the outside through a 

 pneumostome which can be opened and closed. Part of the 

 internal surface of the cavity is richly supplied with blood 

 vessels, forming- ridges which increase about 2-3 times the 

 available respiratory surface (Dahr, 1924). The lung in the 

 shell-bearing species is part of the mechanism for retraction of 

 the animal into the shell. In Helix pomatia I have measured 

 the volume of the lung when the animal was out to 5-7 ml, 

 but when the snail is retracted it can be reduced to less than 

 1/2 ml. In the naked forms the volume is quite small (about 

 0.3 ml in an Arion weighing 10 g). The walls of the lung are 

 muscular, apparently both in the shell-bearing and in the 

 naked forms. 



Dahr made measurements on Arion of about 10 g weight. 

 He found the pneumostome to present generally a diameter 

 between 4 and 6 mm, while the respiratory surface amounted 

 to 6-7 cm 2 . According to his calculations the pressure differ- 

 ence necessary to supply the lung surface with oxygen by 

 diffusion from the outside amounts to about 2 mm Hg, and 

 this shows that even in considerably larger forms and at low 



